The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with steady-state magnetic resonance imaging and will be described with particular reference thereto. It is to be appreciated that the present invention is also applicable to other types of diagnostic imaging and is not limited to the aforementioned applications.
In magnetic resonance imaging, a uniform main magnetic field is created through an examination region in which a subject to be examined is disposed. With open magnetic resonance systems, the main magnetic field is vertical, perpendicular to the subject. With classical bore systems, the main magnetic field is along the head to foot horizontal axis of a prone subject. A series of radio frequency (RF) pulses and magnetic field gradients are applied to the examination region to excite and manipulate magnetic resonances. Gradient magnetic fields are conventionally applied to encode spatial position and other information in the excited resonance.
In steady-state imaging, the repetition time is significantly shorter than the spin relaxation times. That is, the magnetic resonance signals induced by one RF pulse are recycled repeatedly in subsequent repetitions. Typically, an RF excite pulse tips selected dipoles into a transverse plane and decays to zero before application of the next RF pulse. In coherent steady-state imaging, a series of RF pulses are used to tip additional magnetization into the transverse plane and to manipulate previously tipped still decaying magnetization. After each RF pulse, resonance is sampled. Phase-encode gradients, are applied to the main magnetic field before and after each line is read to phase-encode the sampled data and remove the phase-encoding before the next RF pulse. Typically, the phase-encode gradients are stepped in discrete values. Read gradients are applied during readout to encode frequency. The resonance data is collected orderly along a series of linear data lines in k-space. This method of data collection is inefficient. The system reads a line, then rewinds back to zero and reads the next line, and so on. In this manner, the sampling trajectory traverses twice the width of k-space, for each data line sampled. The data matrix and k-space are filled in much like a typewriter filling a sheet of paper. The typewriter does not fill anything in while it is returning to the left margin, and typical steady-state MR imaging techniques do not read as they are rewinding and stepping the phase-encode gradient. This is inefficient because half of the time data is not being read. Further, this method requires high gradients, which strain the gradient amplifiers, and can cause the patient discomfort.
One application of steady-state imaging is imaging of the heart. In cardiac imaging, data is typically gathered over the time span of several cardiac cycles. The gathered data is temporally sorted, and grouped to form multiple images at different phases of the cardiac cycle. Encoding errors can accumulate over time, in particular, phase-encoding errors. These errors can accumulate over time, degrading image quality.
The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance is provided. A subject is disposed in an imaging region and resonances are repeatedly tipped into a transverse plane to establish a steady state. Gradient pulses are applied to define sampling trajectories through a k-space that begin at a point and pass through that point again. The resonance signals are received, sampled, demodulated, and reconstructed.
According to another aspect of the present invention, a magnetic resonance apparatus is provided. A main magnet assembly generates a temporally constant main magnetic field in an imaging region. A gradient subsystem applies gradient pulses to the main magnetic field, defining a sampling trajectory through a k-space. The trajectory oversamples at least one point. An RF system excites resonance, a sampling circuit samples the resonance, an RF receiver receives the resonance, a processor analyzes the demodulated signals, and a reconstruction processor reconstructs the resonance into an image representation.
According to another aspect of the present invention, a method of steady-state magnetic resonance imaging is provided. A sampling trajectory is defined along a closed path having an origin point and an end point. Data is sampled along the path between the origin point and the end point. The data is reconstructed into an image representation. The data sampled at the origin point is compared to the data sampled at the end point.
One advantage of the present invention is that it utilizes efficient data collection scenarios.
Another advantage of the present invention is that it makes use of non-steady-state portions of the resonance data to make steady-state images.
Another advantage of the present invention is that it self corrects phase discrepancies.
Another advantage of the present invention resides in a reduced repetition time.
Another advantage of the present invention resides in gradient and dB/dt requirements.
Another advantage of the present invention resides in its suitability for sliding window reconstructions.
Another advantage of the present invention resides in fractional signal averaging.
Yet another advantage of the present invention resides in the ability to collect dual resolution data in a single sequence.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.